This invention relates to a method for reducing the concentration of one or more pollutants contained in a gaseous mixture. More particularly, this invention relates to a method for reducing the nitrogen oxide content of such a gaseous mixture.
Nitrogen oxides are, of course, generally present in significant amounts in gaseous mixtures such as flue gases which are formed by the combustion of a fossil fuel with air. The amount and type of nitrogen oxides in such flue gases depends on both the fuel nitrogen content and on combustion conditions, and may vary widely, from about 150 ppm to about 1500-2000 ppm. Generally, nitrogen oxide contents are greatest in flue gases produced from solid fuels. Moreover, sulfur oxides will be present when the fuel contains sulfur. In this regard, it should be noted that combustion of conventional high sulfur fuels, such as coal, residuum, and fuel oil, yield flue gases which, typically, contains about 0.1-0.5% by volume of sulfur oxides (SO.sub.x), mostly in the form of SO.sub.2 with small amounts of SO.sub.3. The amount of sulfur oxides in flue gas can be reduced considerably by the use of low sulfur fuels, such as low sulfur coals or low sulfur or desulfurized oils. Such fuels are seldom sulfur-free, however, and, indeed, the sulfur content may be sufficiently high so that treatment of the flue gas to remove sulfur oxides is still necessary or desirable. Generally, some oxygen (typically about 1-8% by volume) will also be present due to the use of excess air in combustion. Moreover, the flue gas may contain small amounts of finely divided suspended particulate matter, such as carbonaceous material and fly ash. Ordinarily there is little or no carbon monoxide or gas phase hydrocarbons in flue gas. The principal constituents of flue gas are nitrogen, carbon dioxide and water vapor.
Processes for the removal of nitrogen oxides, commonly denoted NO.sub.x, from various gases are known. Among the gases treated to remove NO.sub.x are internal combustion engines (particularly automobile) exhaust gases, nitric acid plant tail gas, and flue gas from stationary combustion sources. Various methods for reducing the quantity of NO.sub.x emissions have been proposed, including combustion modification and flue gas treatment. Flue gas treatment methods include catalytic decomposition of NO.sub.x under oxidizing conditions, nonselective catalytic reduction under reducing conditions, selective reduction of NO.sub.x with a suitable reagent such as ammonia under oxidizing conditions, adsorption of NO.sub.x by a solid adsorbent, and absorption of NO.sub.x with a suitable liquid such as aqueous alkaline solution or molten alkali metal carbonate. A comprehensive review of the state of the art is contained in a report by W. Bartok et al, "Systems Study of Nitrogen Oxide Control Methods for Stationary Sources", Report No. GR-2-NOS-69 (PB 192789), Nov. 20, 1969 (prepared under Contract No. PH-22-68-65 for Division of Process Control Engineering, National Air Pollution Control Administration). Section 5 (pages 5-1 to 5-65) in Volume II of this report is of particular interest. For convenience, however, some references, including several which are cited in the foregoing report, will be specifically mentioned here.
It is known that nearly quantitative nonselective catalytic reduction of NO.sub.x under reducing conditons can be achieved, but that decomposition of NO.sub.x into nitrogen and oxygen under oxidizing conditions gives incomplete NO.sub.x removal. See, for example, H. C. Andersen et al, "Catalytic Treatment of Nitric Acid Plant Tail Gas", Ind. Eng. Chem., 53, 199-204 (March 1961); M. Shelef et al, Chemical Engineering Progress Symposium Series, 67, 74-92 (1971). Examples of prior art teaching the selective reduction of nitrogen oxide with ammonia include U.S. Pat. No. 2,975,022 (HNO.sub.3 tail gas; supported Pt group metal catalysts; 150.degree.-400.degree. C.); U.S. Pat. No. 3,008,796 (HNO.sub.3 tail gas; supported iron, cobalt or nickel catalyst; 250.degree.-800.degree. F.); U.S. Pat. No. 3,279,884 (HNO.sub.3 tail gas; oxide of vanadium, molybdenum, or tungsten as catalyst; 150.degree.-400.degree. C.); U.S. Pat. No. 3,328,115 (HNO.sub.3 tail gas; platinum metal catalyst; 150.degree.-400.degree. C.); U.S. Pat. No. 3,449,063 (auto exhaust gas; catalyst of copper oxide on supports such as activated alumina; 250.degree.-800.degree. F.); U.S. Pat. No. 3,599,427 (auto exhaust gas; two-stage process in which NO.sub.x is removed in the second stage, using a CuO or Pt catalyst); and German Patent No. 1,259,298 (flue gas; Fe.sub.2 O.sub.3 --Cr.sub.2 O.sub.3 --CrO.sub.3 catalyst). Attention is also called to Ind. Eng. Chem., 53, 199-204 (March 1961) supra, which discloses the selective reduction of NO.sub.x with ammonia under oxidizing conditions at about 150.degree.-250.degree. C., using platinum, palladium, ruthenium, cobalt or nickel as the catalyst; and Chem. Eng. Progress Symposium Series, 67, 74-92 (1971), supra, which discloses the reduction of NO.sub.x by NH.sub.3 using a noble metal catalyst or nickel at a temperature below 350.degree. C. See also Atmospheric Environment, 6, 297-307 (1972), which discloses reduction of NO with ammonia over barium-promoted copper chromate and over nickel oxide plus copper oxide on gamma alumina catalysts. K. Otto and M. Shelef, The Journal of Physical Chemistry, 76, 37-43 (1972), discuss the reaction of NO and ammonia over pure, low surface area (0.88 m.sup.2 /g) copper oxide at 150.degree.-200.degree. C. See, on the other hand, H. J. Hall et al, Environmental Science and Technology, 5, 320-326 (April 1971) wherein prior processes for catalytic removal of nitrogen oxides from nitric acid plant tail gas by reaction with ammonia was characterized as "unsuccessful".
The selective reduction of nitrogen oxides with hydrogen under oxidizing conditions has also been proposed; see Bartok et al, Report GR-2-NOS-69 (PB 192,789) supra; temperatures used in this case are generally lower than those used with ammonia.
Much of the art on the selective catalytic reduction of nitrogen oxide by ammonia suggests the use of noble metal catalysts, suchh as platinum. Noble metal catalysts are easily poisoned by sulfur, however, and therefore ordinarily cannot be used for treatment of flue gas generated in stationary combustion sources. Even when low sulfur fuel is used, the sulfur oxide content is generally sufficient to poison a noble metal catalyst within a comparatively short time. Also, even in a sulfur-free environment such noble metal catalysts have limited life and are expensive.